We conducted the weekly monitoring (December 2012~April 2013) to evaluate the temporal variation and identification of Stephanodiscus spp. that are generally dominant during winter in the Yeongsan River. Phytoplankton species were identified and counted using the optical microscope and scanning electron microscope (SEM). Phytoplankton in the river were grouped into 6 classes (bacillariophyceae, chlorophyceae, cryptophyceae, cyanophyceae, dinophyceae, euglenophyceae), 30 genus and 41 species. Phytoplankton composition showed high abundance of diatoms in winter and Aulacoseira sp., Cyclotella sp. and Stephanodiscus spp. were dominant. Among the species, Stephanodiscus spp. was relatively abundant compared to other diatom species. Stephanodiscus spp. appeared from December 2012 to April 2013 and their abundance peaked in January. Abundance of diatoms especially peaked ($21,080cells;mL^{-1}$) in January 15, 2013 when Stephanodiscus spp. also bloomed ($20,560cells;mL^{-1}$). The abundances of Stephanodiscus spp. were gradually decreased from March and reached as low as $60cells;mL^{-1}$ in April 26. Cyclostephanos (C. invisitatus), Cyclotella (C. meneghiniana), Discostella (D. pseudostelligera, D. woltereckii) and Stephanodiscus (S. hantzschii, S. minutulus, S. parvus) were classified in the circular diatoms. Abundance of S. hantzschii was extremely high compared to S. minutulus and S. parvus.